There have been used, as a curing agent for polyurethane/urea, aromatic aniline compounds typified by IHARA CUAMINE MT [a trade name for a product of Ihara Chemical Industry Co. Ltd., commonly known as “MBOCA”, 4,4′-methylenebis(2-chloroaniline) (hereinafter, referred to simply as CUAMINE MT)]. When, for example, this CUAMINE MT is actually used, it is melted at one time by the total amount in which it is used on the working day, for reasons of working efficiency and facility, and the melt is gradually used from the start of polyurethane/urea production to the completion of the operation; therefore, part of the melt is kept unused for many hours (about 6 to 13 hours in the production day) and, in some cases, until next morning (until actual use) in the presence of oxygen at temperatures (approximately 120 to 160° C.) which are higher that its melting point (about 100° C.); as a result, CUAMINE MT gives rise to discoloration.
Since discoloration-cause portion in the discolored curing agent or the partial structure thereof remains in the polyurethane/urea produced; therefore, if there is color difference in the curing agent, between the start of operation and the completion of operation, that is, there is discoloration of the curing agent, there is color difference as well in the polyurethane/urea produced between the start of operation and the completion of operation. Therefore, there is no problem when the usage of the polyurethane/urea is not affected by the color thereof; however, when the usage of the polyurethane/urea is affected by the color thereof, the above-mentioned discoloration of the curing agent with the passage of time becomes a major problem which cannot be neglected.
Meanwhile, it was proposed to add, to an aromatic amine, an organic phosphorus compound selected from phosphines, phosphorous acid esters and phosphoric acid esters, to stabilize the aromatic amine. This stabilization includes an aspect of the inhibition of discoloration.
Patent Literature 1: JP-B-1980-31135
This Patent Literature, however, makes no mention as to whether or not the above technique can inhibit the discoloration of methylenebisaniline compounds such as CUAMINE MT and the like when the compounds have been used in such a manner as mentioned above.